The Essence of Scala

Wednesday 3 February 2016

Martin Odersky

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What do you get if you boil Scala on a slow flame and wait until all
incidental features evaporate and only the most concentrated essence
remains? After doing this for 8 years we believe we have the answer:
it’s DOT, the calculus of dependent object types, that underlies Scala.

A paper on DOT will be
presented in April at Wadlerfest,
an event celebrating Phil Wadler’s 60th birthday. There’s also a prior
technical report (From F to DOT)
by Tiark Rompf and Nada Amin describing a slightly different version
of the calculus. Each paper describes a proof of type soundness that
has been machine-checked for correctness.

The DOT calculus

A calculus is a kind of mini-language that is small enough to be
studied formally. Translated to Scala notation, the language covered
by DOT is described by the following abstract grammar:

The full calculus adds to this syntax formal typing rules that
assign types T to terms t and formal evaluation rules that
describe how a program is evaluated. The following type soundness
property was shown with a mechanized, (i.e. machine-checked) proof:

If a term t has type T, and the evaluation of t terminates, then
the result of the evaluation will be a value v of type T.

Difficulties

Formulating the precise soundness theorem and proving it was unexpectedly hard,
because it uncovered some technical challenges that had not been
studied in depth before. In DOT - as well as in many programming languages -
you can have conflicting definitions. For instance you might have an abstract
type declaration in a base class with two conflicting aliases in subclasses:

Now, if you combine Sub1 and Sub2 in trait Bad you get a conflict,
since the type A is supposed to be equal to both String and Int. If you do
not detect the conflict and assume the equalities at face value you
get String = A = Int, hence by transitivity String = Int! Once you
are that far, you can of course engineer all sorts of situations where
a program will typecheck but cause a wrong execution at runtime. In
other words, type soundness is violated.

Now, the problem is that one cannot always detect these
inconsistencies, at least not by a local analysis that does not need
to look at the whole program. What’s worse, once you have an
inconsistent set of definitions you can use these definitions to
“prove” their own consistency - much like a mathematical theory that
assumes true = false can “prove” every proposition including its own
correctness.

The crucial reason why type soundness still holds is this: If one
compares T with an alias, one does so always relative to some pathx that refers to the object containing T. So it’s really x.T =
Int. Now, we can show that during evaluation every such path refers
to some object that was created with a new, and that, furthermore,
every such object has consistent type definitions. The tricky bit is
to carefully distinguish between the full typing rules, which allow
inconsistencies, and the typing rules arising from runtime values,
which do not.

Why is This Important?

There are at least four reasons why insights obtained in the DOT
project are important.

They give us a well-founded explanation of nominal typing.
Nominal typing means that a type is distinguished from others
simply by having a different name.
For instance, given two trait definitions

we consider A and B to be different types, even though both
traits have the same parents and both define the same members.
The opposite of
nominal typing is structural typing, which treats types
that have the same structure as being the same. Most programming
languages are at least in part nominal whereas most formal type systems,
including DOT, are structural. But the abstract types in DOT
provide a way to express nominal types such as classes and traits.
The Wadlerfest paper contains examples that show how
one can express classes for standard types such as Boolean and List in DOT.

They give us a stable basis on which we can study richer languages
that resemble Scala more closely. For instance, we can encode
type parameters as type members of objects in DOT. This encoding
can give us a better understanding of the interactions of
subtyping and generics. It can explain why variance rules
are the way they are and what the precise typing rules for
wildcard parameters [_ <: T], [_ >: T] should be.

DOT also provides a blueprint for Scala compilation. The new Scala
compiler dotty has internal data structures that closely resemble DOT.
In particular, type parameters are immediately mapped to type members,
in the way we propose to encode them also in the calculus.

Finally, the proof principles explored in the DOT work give us guidelines
to assess and treat other possible soundness issues. We now know much
better what conditions must be fulfilled to ensure type soundness.
This lets us put other constructs of the Scala language to the test,
either to increase our confidence that they are indeed sound, or
to show that they are unsound. In my next blog I will
present some of the issues we have discovered through that exercise.